This invention relates to agents to prevent or minimize polymer scale deposition and a method to prevent polymer scale deposition during the polymerization of monomers which comprise an ethylene type double bond.
Polymer manufacturing processes involving polymerizing monomers in a polymerization vessel have the problem of polymer scale deposition on the vessel inner walls. Such depositions reduce the heat transfer of the vessel walls, resulting in lowering of productivity. Further, any released scale entering into the polymer product results in a reduction of polymer quality. The removal of such scale usually requires a great deal of labor and time. The work necessary to remove the scale causes a very serious health problem to the workers, since unreacted toxic monomer is absorbed in the polymer scale.
During the polymerization of monomers with an ethylene type double bond, a conventional method to prevent polymer scale deposits on the inner walls of the reactor is to employ on the walls a coating of appropriate substances which prevent polymer scale deposition. Such appropriate substances already known are, for example, specific polar compounds (JP70-30343), dyes and pigments (JP70-30835), aromatic amine compounds (JA76-50887), and products from the reaction between phenolic compounds and aromatic aldehydes (JA80-54317). These substances are effective to prevent polymer deposits during the polymerization of vinyl halogenide monomers such as vinyl chloride or of a monomeric mixture composed mainly of vinyl halogenides.
When the monomers have an ethylene type double bond, such as styrene, alpha-methylstyrene, acrylic acid ester, and acrylonitrile, such substances are no longer effective for scale deposit prevention. Monomers with an ethylene type double bond can be expressed by the formula (I): EQU CH.sub.2 .dbd.CXY (I)
where X is a hydrogen atom or a methyl group and Y is a hydrogen atom, an alkyl group, or a group of the formulae --COOH, --COOM (wherein M is an alkali metal or ammonium ion), --COOR (wherein R, here and below, is an alkyl group), --OCOR, --OR, --CN, --C.sub.6 H.sub.5, --C.sub.6 H.sub.4 Z (wherein Z is a hydrogen atom, --OH, --CH.sub.3, or a group of the formula --CH.dbd.CH.sub.2), or a group of the formula --CH.dbd.CH.sub.2. These monomers tend to dissolve coatings formed by such conventional polymer scale deposit inhibitors. As a result, they lead to a loss of a portion or all of the coating film and eventually to a loss of the scale prevention effects. Styrene, alpha-methylstyrene, acrylic acid ester, and acrylonitrile are monomers which have extremely large solubility towards such coating films; therefore, it is difficult to prevent polymer scale deposition effectively during the polymerization of these compounds. Particularly when the coated polymerization reactor is used repeatedly, it becomes difficult to achieve the originally intended scale prevention effect. Therefore, usage of the conventional polymer scale deposit inhibitors necessitates that a fresh coating film be prepared for every polymerization batch, which makes improving productivity difficult.
Inner walls of polymerization vessels made of stainless steels and other types of steels tend to deposit scales even easier than glass-lined walls.